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    <title>Document</title>
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    <style>
        div{
            height: 10000px;
        }
        img{
            position: absolute;
            top: 170px;
        }
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    <div></div>
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        window.onscroll = function(){
            var scrtop = document.documentElement.scrollTop || document.body.scrollTop
            utils.bfmove(img,'top',scrtop+170)
        }
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